The use of holograms for recording and displaying information is being applied to an ever-widening range of fields including advertising displays, head-up displays and the like.
The recording of a hologram of an object is accomplished by establishing an interference pattern between an object wavefront scattered by the object and a reference wavefront, the two wavefronts being mutually coherent. An image of the object is reconstructed from the hologram with an illumination wavefront which is usually directed onto the hologram at the same angle as the reference wavefront. For complete illumination of the hologram, the reference wavefront (usually generated from a point source of light) must be located a relatively large distance from the hologram. The resulting playback geometry occupies a large distance in the direction perpendicular to the hologram. In many applications, this space requirement complicates the installation of a system which relies on holographic readouts.
When using a transmission hologram the brightness of the light transmitted through the hologram may exceed the brightness of the image formed by the diffracted energy, making the image difficult to see. The transmitted light may also exceed safe light levels since it does not originate from a diffusely reflecting surface. In addition, ambient light shining on the hologram may be diffracted by the hologram, forming objectionable stray light. This diffracted ambient light can interfere with the performance of the holographic readout system.
Consequently, the need exists for improvements in developing compact holographic systems which will offer greater flexibility in their installation, improved contrast with the illuminating light and low levels of diffracted stray light.